diff --git a/revocation/shachain.go b/revocation/shachain.go
index 39551128a..57a2e8f5a 100644
--- a/revocation/shachain.go
+++ b/revocation/shachain.go
@@ -9,6 +9,7 @@ type chainFragment struct {
 }
 
 // HyperShaChain...
+// * https://github.com/rustyrussell/ccan/blob/master/ccan/crypto/shachain/design.txt
 type HyperShaChain struct {
 	lastChainIndex uint64